Description of the Prior Art:
Most oil and gas wells being drilled are logged at least once during the drilling. In conventional logging, the drill pipe will be pulled, and one or more instruments are lowered on conductor cable into the open hole to measure earth formation characteristics.
Deviated wells that may incline up to 65 degrees or more are common, particularly at offshore locations. A number of wells may be drilled from a single platform. These wells present difficult problems for logging. The logging instrument may not be able to reach bottom due to the inclination. Also, the cable dragging against the curved portion of the well may create a slot or key, causing the instrument to become stuck.
A recent technique has been developed to log deviated wells. In this technique, when it is desired to log a portion of the well, the drill pipe is pulled. The logging instruments are located in special housings and secured to the lower end of the drill pipe. The drill pipe is then run into the well until it is located near the upper end of the zone to be logged. Then, a side entry sub is secured to the upper end of the drill pipe. The side entry sub has a passage extending through its sidewall for cable to pass. A latch is threaded through the sidewall passage, and a packing is placed around the sidewall passage. The latch is pumped down with drilling fluid pressure into electrical engagement with the instruments at the bottom of the well. The cable is placed under tension, and a clamp clamps the cable to the side entry sub.
The string is then lowered farther into the well. Normally, tie wraps will be used to secure the cable to the exterior of the drill pipe as the string is lowered into the well. When the bottom of the well is reached, the side entry sub may be several thousand feet below the surface, but it will still be located in casing.
To log the well, the drill string is then pulled upward. The instruments are energized while each stand is pulled to log the well. The cable at the surface is simultaneously taken up. When the side entry sub again reaches the surface, the clamp is removed and tension is applied to the cable to cause the latch to release from its connection to the logging instruments. The cable is then pulled from the drill pipe and the drill string is then removed normally.
Serious problems occur if the drill pipe becomes stuck in the well while the logging cable is in the drill pipe. The clamp is of a shear release type. However, due to the friction between the cable and drill pipe, and the tie wraps, the clamp may not shear as required. Excessive pulling on the cable may result in it parting at a point above the clamp. A chemical cutter or a jet charge can be lowered into the drill pipe to cut the cable, but it can be cut only at the side entry sub. This still leaves the cable in the drill pipe below the side entry sub.
With the cable in the drill pipe, a stuck point indicator can't be lowered into the drill pipe to indicate where the pipe is stuck. A backoff tool can't be lowered into the drill pipe to back off the drill pipe. There is a possibility that the well would have to be abandoned with the drill string in the well.